142 



PELVIS. 



by the line P c, and the line of gravity of 

 the trunk, ca, and is doubtless considerably 

 increased by the resisting tension of the an- 

 terior abdominal muscles acting through the 

 extensors of the thigh on the femoral sup- 

 ports in the line c b, b d. 



In the foregoing dispositions of the pelvic 

 structure, the office fulfilled by the sacrum is 

 so compendious and important as to call for 

 particular attention to the shape and position 

 of that bone, and the manner in which it is 

 articulated with the ilia, so as to be at once 

 firm and strong as a keystone, yielding as a 

 spring, and moveable as a jointed bone. And 



we shall find all these requirements beauti- 

 fully provided for. 



First. The sacrum is wedge-shaped, when 

 viewed at its anterior aspect, narrowing from 

 above downward, especially along the surface 

 immediately between the lower portions of its 

 iliac articular surfaces in the plane b, e (fig. 

 88. A), which are inclined to each other at 

 an angle varying from 1 5 to 30, and aver- 

 aging about 20. When viewed from above at 

 its base, as in fig. 89. page 144.), it also pre- 

 sents a wedge shape with the base directed ante- 

 rior/^, the lateral masses of the base becoming 

 narrower from before backwards ; so that the 



Fig. 88. 



a 



A, diaqram of the sacral auricular facet, natural size and placed as in ihe erect position of the body, with 

 the lines of tension and pressure, a, centre of action of the posterior deep ligaments; dfe,arc of sacral 

 groove, forming a quadrant with the lines of tension, a d, a e; d e, chord of the arc; d f, e f, chords 

 of the semi-arcs ; d g e, sacro-vertebral angle ; d b e, triangle forming the sacral " voussoir ;" b and {e, 

 position of sacral " joggles." 



B, diagram of a transverse section of the pelvis in the line of the sacral a.ris, posterior view'; a c b, angle 

 of vertical sacral wedge ; d, e, depth of sacro-iliac articulations ; f, iuterosseous sacro-iliac ligaments. 



sacrum appears to be a double wedge, having 

 its broadest part at the border of junction of 

 the base with the anterior surface, and tapering 

 from this point, both upwards and backwards 

 and downwards and backwards. 



Hence it has been stated by Cruveilhier to 

 be liable to dislocation downwards and for- 

 wards, from the want of bony support in that 

 direction. But the sacrum, in the erect po- 

 sition of the body, is placed, not vertically, 

 but obliquely, with its base directed more 

 forwards than upwards, and its anterior sur- 

 face more downwards than forwards, so that 

 the upper limb of the auricular surface is 

 placed nearly vertically, and the lower nearly 

 horizontally, as seen in fig. 88. A. The di- 

 minished breadth of the base of the sacrum 

 posteriorly is due to the bevelling of the 

 lateral surfaces for the implantation of the 

 deep posterior and interosseous ligaments, as 



seen in fig. 88. (B, c),the bone not being here 

 in apposition with the overhanging iliac tuber- 

 osities, the area of absolute contact being 

 confined to the auricular surfaces themselves. 

 Again, the increased breadth of the anterior 

 surface at the auricular angle c {fig. 89.), 

 will be found, on careful inspection, to depend 

 upon the presence of the pointed projections 

 on each side, before described as received 

 into corresponding depressions on the au- 

 ricular surfaces of the ilia, which latter, being 

 circumscribed below by a raised border, cause 

 the sacrum to bite on the ilia here to a consider- 

 able extent, forming what is called, in engi- 

 neering nomenclature, a "joggle" to the sacral 

 " voussoir." By the iliac support thus re- 

 ceived, . the position of the sacrum is well 

 protected against pressures, coming either 

 directly downwards in the line d, b (fig. 86. A), 

 or downwards and forwards in the direction of 



